Metallized film capacitors having electric properties of high withstand voltage and low loss have drawn attention recently, and there has also been a noticeable trend to adopt such metallized film capacitors having an extremely long lifetime, in association with the demand for maintenance-free products in the market.
Metallized film capacitors are generally categorized into those including a metal leaf as an internal electrode and those including a vapor-deposited metal which is provided on a dielectric film and used as an internal electrode. Of these metallized film capacitors, the metallized film capacitors including a vapor-deposited metal as an internal electrode, which occupies a smaller area than that of the metal leaf electrode and therefore can reduce the size and weight thereof, have been conventionally used. (See JP 10-149939 A, for example).
FIG. 8 is a view illustrating a structure of this type of conventional metallized film capacitor 10, and FIG. 9 is a side view of the metallized film capacitor 10. While in FIG. 8 and FIG. 9, the thickness β of a dielectric film 20P, 20N is shown greater than the actual scale for the sake of clarity, the thickness β of the dielectric films 20P and 20N is actually sufficiently smaller than the width α of a vertical pattern 26P, 26N. For example, the thickness β of the dielectric films 20P and 20N is approximately 1/50 to 1/300 the width α of the vertical pattern 26P, 26N, and is more preferably 1/100 to 1/300 the width α of the vertical pattern 26P, 26N. Further, in the following description, when it is not necessary to discriminate between the P-polarity side and the N-polarity side, the letters “P” and “N” will be omitted from reference numerals.
A metallized film 12 forming the metallized film capacitor 10 is formed by vapor deposition of a metal such as aluminum onto one side of a dielectric such as a polypropylene film. A laminate formed by stacking this metallized film 12 is cut to a predetermined length. On each of both ends in the width direction of an element obtained by such cutting, a metallized contact electrode 14 is disposed as an external electrode.
On one side of each metallized film 12, an edge margin portion 22, a protection mechanism portion 24, a vertical pattern 26, and a solid electrode 28 are sequentially arranged in this order in the width direction. The edge margin portion 22 is a portion of the metallized film 12 at one edge where no metal is vapor-deposited and is provided to extend across the metallized film 12 in the length direction. The protection mechanism portion 24 including a plurality of segmented electrodes 30 arranged therein is disposed adjacent to this edge margin portion 22. The segmented electrodes 30 are segmented by an insulating slit pattern 32 and are simultaneously connected with each other via a fuse 34.
The solid electrode 28 is a portion having a metal vapor-deposited on the entire surface thereof and is contiguous to the metallized contact electrode 14. The vertical pattern 26, which is a slit pattern extending across the metallized film 12 in the length direction, is provided between the solid electrode 28 and the protection mechanism portion 24. Several fuses 34 are provided along the vertical pattern 26, and the solid electrode 28 and the segmented electrodes 30 are interconnected via the fuse 34.
The metallized films 12 are stacked such that the locations of the edge margin portions 22 in the width direction are alternately opposite. In other words, the metallized films 12 are stacked such that the metallized contact electrodes 14 of different polarities connect to the solid electrodes 28 in an alternating manner.
The conventional metallized film capacitor 10 configured as described above has a self-healing function specific to the metal vapor deposition electrode (which means a property of restoring the function of a capacitor by evaporation and dispersion of the internal electrode around a defect portion, and is generally referred to as “self-healing property”), and further has a property which, when a large amount of electric current flows in the segmented electrodes, due to deficiency of the segmented electrodes, dispersion of the fuse occurs to thereby break electrical connection of the deficient segmented electrodes, thereby restoring a normal state of the electric current. As such, the metallized film capacitor 10 has a characteristic nature against dielectric breakdown and therefore has excellent reliability.
As described above, the metallized film capacitor 10 illustrated in FIGS. 8 and. 9 is formed by cutting the laminate made of the metallized films 12 to a predetermined length. Normally, a cut section formed by such cutting is provided with an insulating film for securing insulation. While no problems would arise if this insulating film has a sufficient thickness such that insulation is reliably secured, there is a possibility that dielectric breakdown would occur in the cut section if this insulating film is thin or the insulating film is deficient.
Specifically, if the insulating film on the cut section is thin, the electric current flowing into the solid electrode 28P on the P-polarity side flows into the segmented electrode 30N located immediately under the solid electrode 28P along the cut section, as indicated by dashed lines in FIG. 9. In this case, due to dispersion of the fuses around the segmented electrode 30N, the segmented electrode 30N is electrically disconnected from a segmented electrode within the same plane. However, because the creepage distance between this segmented electrode 30N and the segmented electrode 30P located immediately above the segmented electrode 30N (the thickness β of the dielectric film 20) is very small, the segmented electrode 30N is not electrically disconnected from the segmented electrode 30P. As a result, the electric current also flows to the segmented electrode 30P located immediately above the segmented electrode 30N along the cut section. Further, according to a similar principle, this electric current further flows to the segmented electrode 30N located immediately under the segmented electrode 30P, and further to the segmented electrode 30P located immediately above the segmented electrode 30N, and finally to the solid electrode 28N located immediately under the segmented electrode 30P. This results in short circuit between the solid electrode 28P on the P-polarity side and the solid electrode 28N on the N-polarity side, which may lead to dielectric breakdown between these solid electrodes 28P and 28N.
It is therefore an advantage of the present invention to provide a metallized film capacitor which is designed to prevent such dielectric breakdown on a cut section more reliably.